An epidemic is the occurrence of an illness among a large number of people in the same geographical area at the same time. Bacterial epidemics have probably been part of the lives of humans since the species evolved millions of years ago. Certainly by the time humans were present, bacteria were well established.

On example of a bacterial epidemic is the plague. Plague is caused by the bacterium Yersinia pestis. The bacterium lives in a type of rodent flea and is transmitted to people typically via the bite of the flea. People who come into contact with an infected animal or a flea-infested animals such as a rat can also contract the disease.

Plague has been a scourge on human populations for centuries. In the Middle Ages, the so-called Black Plague (Bubonic plague) killed millions of people in Europe. The crowded living conditions and poor sanitation that were...